Aerospace vehicles (spacecraft or high speed aircraft) encounter high skin temperatures because of resistance with the surrounding atmosphere and develop large temperature gradients across the skin. To protect these vehicles, several thermal protection systems have been used combining active and passive cooling techniques. Two problems are faced. First, the skin material must be able to withstand the high temperature. Second, the large gradient across the skin must be controllable to maintain the compartment within the vehicle at a habitable temperature.
For spacecraft, the high skin temperatures require ceramic skins and the gradient problem is resolved simply by the short duration of the exposure (during lift-off or re-entry). Low density ceramics are required to reduce the vehicle weight, yet the insulation must be sufficiently, strong and durable to withstand the rigors of space travel. In copending U.S. patent application Ser. No. 698,496, Anna Bendig (Baker), now U.S. Pat. No. 5,041,321, describes a fiberformed ceramic insulation possessing desirable physical and chemical properties for this space vehicle application.
For high speed aircraft, the problem that is most difficult to solve is the thermal gradient Longer flights, even at lower maximum skin temperatures, can create internal temperatures that approach the skin temperature unless cooling is provided A passive thermal protection system for the X-20 aircraft used a water-saturated, open-cell polyurethane foam (sponge) between the metallic skin and the inner compartment wall. Cooling capitalized on the large latent heat of vaporization of the retained water.
This sponge "water wall" concept presented numerous problems. The foam could degrade or decompose after extended exposure to high temperatures, especially if it dried out. Minor decomposition posed outgassing problems; extensive decomposition posed risks of total system failure. The water in the foam had to remain dispersed throughout the foam, even during high accelerations, for the system to operate effectively. Accordingly, a gelling agent was used to thicken the water. To be compatible with the foam, cyanides were often used in the gelling agent, posing toxicological problems both during installation and use. Repair and inspection of the system (which was essential because of the risk of decomposition of the foam) required removal of the skin. Such inspection hindered reusability of the vehicles because it was time-consuming and required skilled labor The foam was difficult to saturate, and local discontinuities in the water concentration posed the threat of system failure Accordingly, exotic radiographic inspection techniques and water-filling techniques were developed to ensure even and complete dispersion of the water throughout the foam.
In U.S. Pat. Nos. 4,482,111 and 4,592,950, LeTouche of Aerospatiale describes thermal protection or dissipation screens that include a hot wall and a cool wall separated by an insulating layer. The layer includes a stratified hydrating element, a stratified overheating element, and a porous stratified refractory element. The LeTouche system maintains the temperature of the cool wall at or below the vaporization temperature of the hydrating element through the evaporization of the element. In U.S. Pat. No. 4,482,111, LeTouche uses a supple or flexible plastic or ceramic material to hold a gelled liquid, and supplements the cooling achieved by evaporation by including carbon in the refractory element to react with the steam that is created to form water-gas in an endothermic (heat absorbing) reaction.
In U.S. Pat. No. 4,592,950, LeTouche incorporates a reactive substance into the refractory element and a membrane that is impermeable until a predetermined temperature is reached or exceeded. The LeTouche systems are passive devices to protect flight recorders and other critical elements from catastrophic events, and are not designed for reuse or recharging.
Active cooling (refrigeration) requires complicated mechanical systems that are expensive to build and expensive to maintain. Such systems generally are not as reliable as passive systems. With active cooling, it is difficult to avoid localized hot spots without including sophisticated sensors and plumbing in the system.
These problems are substantially overcome with the passive thermal protection system of the present invention.